Abstract
X-ray astronomy began in 1948 with the first detection of X-rays from the Sun. Astronomical X-ray observations need to be performed from high-altitude rockets and satellites because the Earth's atmosphere absorbs X-rays. Currently about 100,000 X-ray sources are known all over the sky. The Sun is by far the strongest source. The outermost solar atmosphere, the corona, emits X-rays due to its high temperature of a few million K. Solar X-ray emission is highly variable. Eruptions lead to variations of the X-ray flux on time scales of minutes. The average X-ray flux varies with the 11-year sunspot cycle by a factor of about 1000. Solar X-rays have a profound influence on the Earth's upper atmosphere.
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